1. Field of Invention
This invention relates to radiation shielding for the targeting assembly of a cyclotron or particle accelerator used in a radiopharmaceutical or radioisotope production system. More specifically, the present invention is related to a closure which is mounted on the housing of a particle accelerator or cyclotron, and which serves as radiation shielding for, and provides access to, such targeting assembly.
2. Description of the Related Art
Positron Emission Tomography (PET) is a powerful diagnostic tool which allows the imaging of biological functions and physiology. PET utilizes short-lived radioactive isotopes, commonly referred to as tracers, which are injected into a patient's body. These radioisotopes are produced by radioisotope production systems which incorporate particle accelerators or cyclotrons. The particle accelerators produce radioisotopes by accelerating a particle beam and bombarding a target material. The typical particle accelerator used for producing PET radioisotopes includes a targeting assembly which is accessible from outside of the housing of the accelerator, and generally through an access opening in the housing, such that the target material can be replaced and such that maintenance can be performed on the targeting assembly. In order to protect those operating and maintaining the accelerator from the radiation emanating from the accelerator, the entire accelerator is placed in a shielded enclosure. For example, such shielded enclosures often take the form of a shell which surrounds the accelerator or cyclotron, with the shell being provided with movable portions or doors to provide access to the accelerator. The shielded enclosures typically include a high-Z shielding material, such as lead, adjacent the accelerator to moderate neutron energy and shield against gamma radiation, and a low-Z outer shielding, such as concrete, to absorb neutrons and, again, to provide gamma shielding. Commonly, the high-Z shielding defines a greater thickness proximate the targeting system of the accelerator given the neutron energy typically emanating therefrom. Generally, such shielded enclosures provide the only shielding about the targeting assembly of the accelerator such that when the shielded enclosures are removed or opened the targeting assemblies are accessible, but unshielded. Further, typical shielding enclosures for particle accelerators have a gap greater than one, inch (>1″) between the shielding and the accelerator/target assembly. This is due to the manufacturing tolerances of the shielding materials involved, and the methods for shield motion. Neutrons can be transported through these gaps without being moderated, allowing higher radiation doses outside the shield assembly.
An example of one approach to providing shielding for an accelerator used in conjunction with a radioisotope production system is disclosed in U.S. Pat. No. 6,392,246 B1. The apparatus disclosed therein provides an outer housing which shields not only the accelerator, but various other components of the radioisotope production system. Further, U.S. Pat. No. 5,037,602 discloses a radioisotope production facility, and discusses the need for thick shielding around the accelerator to confine radiation. See also, U.S. Pat. Nos. 6,433,495 B1; 5,874,811; 5,482,865; and 4,646,659.
Radioisotope production systems are commonly located in hospitals and other healthcare facilities such that the radioisotopes are readily available for use in medical imaging. Accordingly, it is imperative that proper radiation shielding be provided to protect not only the operators of the system and the medical staff, but the public. However, the need for thick radiation shielding around the accelerator tends to make radioisotope production systems large, space consuming systems, and the shielding tends to be very heavy. The size and weight of the radioisotope production systems tends to limit the nature of the facilities in which the systems can be placed, and often the construction of special facilities to accommodate the systems is necessary. Thus, it is advantageous to limit the thickness of the shielding surrounding the accelerator to the extent that it can be done without compromising the effectiveness of the shielding. Further, particularly where the radioisotope production system is placed in a healthcare facility, the exposure of the targeting system when the shielded enclosure surrounding the accelerator is removed can be particularly problematic. For example, where access to components of the accelerator other than those associated with the targeting system is required, the removal or the opening of the shielded enclosure leaves the targeting system unshielded, thereby unnecessarily increasing the level of radiation emanating from the accelerator. Additionally, it is advantageous to make shielding that conforms more closely to the accelerator and target envelope, to force the moderation of initially energetic neutrons.